The field of the invention is collimating lenses for X-ray imaging systems and, more particularly, such collimating lenses which utilize a "folded" optical path.
In X-ray imaging systems, an image intensifier tube receives X-rays that have passed through a subject. The image intensifier tube converts the X-rays into an optical image by means of a surface coated with suitable phosphors. The image intensifier tube is usually positioned vertically, for example over a human patient laying prone. The optical output of the image intensifier tube is then obtained through an optical output aperture in the top of a case enclosing the image intensifier tube. The image intensifier tube itself is a hermetically sealed vacuum tube, with an output phosphor surface positioned just below the optical output aperture. A collimating lens is disposed directly over the optical output aperture, and includes an array of lenses to focus the optical output of the image intensifier tube into an image head for subsequent processing of the optical output.
Because of the critical importance of high quality optics, in-line, or axial collimating lenses have been overwhelmingly preferred in prior art X-ray imaging systems (for example, see U.S. Pat. No. 4,980,905). The axial collimating lenses in popular use utilize multiple individual lenses, precision aligned with respect to each other in a straight cylindrical lens housing. X-ray imaging requires extremely high quality, low distortion optics so as to preserve as much detail as possible in the image being processed. In order to achieve that image quality, it is necessary to utilize fixed optical elements in the collimating lens. That is, the individual lenses which are used in the collimating lens must be fixed with respect to each other. As a result, the entire collimating lens is moved with respect to the image intensifier optical output aperture to achieve optimum focus. In that case, focus is achieved by moving the entire lens assembly axially with respect to the optical output aperture. Since the direction of the optical path in an axial collimating lens is invariant with respect to axial rotation, lateral axial movement of the lens assembly is readily achieved by conventional screw threads and simple rotation of the collimating lens assembly. Thus, the conventional axial collimating lenses with in-line, screw thread focusing, have been satisfactory for most prior X-ray imaging systems.
One problem that has arisen in the art is that image intensifier tubes are now being utilized which are physically larger, e.g. taller, than their contemporaries. As a result, use of axial collimating lenses necessitates placement of the image head at a correspondingly greater height. The increased height of the image head in turn greatly complicates access to the image head. Since access to the image head is routinely required, for example, to retrieve exposed film, the greater height of the image head is a significant detriment. Also, a correspondingly greater ceiling height is required for the room which is to contain the X-ray apparatus, which may limit the locations in which the X-ray apparatus may be employed.
To overcome the height problems associated with the larger image intensifier tubes, the use of a folded optic collimating lens has been attempted in prior systems. Folded lens optics are generally known, which utilize an angled mirror to redirect the optical path. For example, a mirror at 45.degree. results in a 90.degree. change in the direction of the optical path. The folded optics collimating lens re-directs the optical output of the image intensifier tube at a 90.degree. angle so that the image head may be placed aside, rather than above, the image intensifier tube. In that way, the image head is lower and easier to access, and the overall height of the X-ray apparatus is reduced.
The folded collimating lens assembly includes separate input and output lenses on opposite sides of the mirror. In that case, focusing by means of a conventional screw threaded lens assembly can be applied to only one of the input or output lenses, and will necessarily change the relative, internal spacing between the input and output lenses. While it is still possible to achieve focus in that manner, the resulting image quality is substantially degraded over that which is achieved by lens assemblies with fixed dimensions between the internal lens elements. Consequently, conventional screw thread focusing of a folded optics collimating lens is not practicable.
Focusing of prior folded optics collimating lenses has therefore required adjusting the axial lateral spacing between the collimating lens input and the image intensifier output aperture. This process is further complicated in that it is not practical to permit rotation folded optics collimating lens assembly; the output of the folded optics collimating lens must remain directed towards the image head. As a result, focusing of prior folded optics collimating lenses has involved essentially trial and error, manual adjustment by the use of shims, etc., to achieve the desired focus. Clearly, this is a time consuming, painstaking process. In addition, optimum focus is often not achieved, or is at least difficult to verify, due to the inherent inaccuracies of the prior focusing methods.